Embodiments of the inventive concepts relate to a member for a gas sensor, a gas sensor using the same, and a method of manufacturing the same. More particularly, embodiments of the inventive concepts relate to a porous semiconductor metal oxide complex nanofiber functionalized by uniformly distributing porous first metal oxide particles, including metal nanoparticle catalysts synthesized using a hollow metal-organic framework formed by combining metal ions with organic ligands, in the inside and on a surface of a second metal oxide nanofiber, a member using the same, a gas sensor using the same, and a method of manufacturing the same.
As awareness to health increases, a semiconductor metal oxide based gas sensor is being actively developed as a sensor technique capable of rapidly detecting various harmful environmental gases and of early providing harmful information or a sensor technique having high sensitivity and high selectivity to early monitor a health symptom of a human body. In particular, researches are being actively conducted for a technique capable of increasing sensitivity and selectivity with respect to a specific gas by combining a catalyst with a sensing material based on the semiconductor metal oxide. A semiconductor metal oxide based gas sensor senses a gas by using an electrical resistance variation of the semiconductor metal oxide which occurs by surface reaction occurring when a specific gas is adsorbed on and detached from a surface of the semiconductor metal oxide material. Since the semiconductor metal oxide based gas sensor analyzes a ratio of a resistance in the specific gas to a resistance in air to quantitatively sense the specific gas, a sensor system may have a simple structure and a small size and may easily work together with another device. Thus, researches are being actively conducted for a technique capable of applying the semiconductor metal oxide based gas sensor to a mobile or wearable device. In addition, the semiconductor metal oxide based gas sensor is applied in various fields such as a harmful environmental gas alarm, an alcohol drinking detector, an atmospheric pollution detector, and a sensor for sensing a terror gas. In particular, a healthcare exhalation sensor capable of early diagnosing a specific disease by sensing a biomarker gas is spotlighted. Various biomarker gases (e.g., acetone, ammonia, nitrogen monoxide, sulfureted hydrogen, toluene, and pentane) exist within a breath exhaled out from a mouth through the lungs of a human body. These gases are reported as biomarker gases for diabetes, a kidney disease, asthma, foul breath, a cancer of the lungs, and a disease of the heart.
Since the breath exhaled through the lungs of the human body includes hundreds of various gases mixed with each other, a specific biomarker gas should be selectively sensed. In addition, a biomarker gas included in the exhaled breath of the human body has a very low concentration ranging from 10 parts per billion (ppb) to 10 parts per million (ppm), and thus a gas sensor capable of accurately sensing a gas having a concentration of about 10 ppb should be developed to sense the biomarker gas. Moreover, the size of the gas sensor should be reduced to use the sensor as a real-time sensing device, and a response time and a recovery time of the gas sensor should be shorter than several seconds. The response time of the gas sensor is a time for which the gas sensor responds to the specific gas, and the recovery time of the gas sensor is a time for which the gas sensor recovers to an initial state in air. However, since the semiconductor metal oxide based gas sensor uses the principle which detects the electrical resistance variation according to the surface reaction occurring when the specific gas is adsorbed on and detached from the surface, the selectivity of the gas sensor reacting with the specific gas is reduced and the gas sensor is difficult to measure the gas having a very low concentration of several ppb. Thus, a sensing material for a gas sensor, which has high sensitivity and high selectivity, should be developed to use the semiconductor metal oxide based gas sensor as the healthcare exhalation sensor.
To allow the semiconductor metal oxide based gas sensor to have the high sensitivity and the high selectivity, various researches are being actively conducted for application and synthesis of various nanostructure based sensing materials including a nanoparticle, a nanofiber, a nanotube, a nanocube, and a hollow nanostructure. The nanostructure based sensing material has a high surface area reacting with gases, which is larger than that of a thick film. Thus, the semiconductor metal oxide based gas sensor has higher sensitivity by using the nanostructure when the semiconductor metal oxide based gas sensor uses the surface reaction between the semiconductor metal oxide material and gas molecules. In addition, when the sensing member has the hollow or porous structure, gases are easily diffused into the sensing material. Thus, the sensing material may have higher sensitivity and a higher reaction rate. In particular, a surface area of a porous nanofiber having a one-dimensional structure is six times or more greater than that of a thin layer structure, and gases are easily diffused into the porous nanofiber. Thus, the sensing member may have higher sensitivity and a higher reaction rate. In addition, when a catalyst is coupled or bonded to the porous nanofiber, the catalyst can be coupled or bonded to both an inner surface and an outer surface of the porous nanofiber shell. In other words, the porous nanofiber has a large surface area at which the catalyst reacts with a gas, and thus the sensing member including the porous nano fiber may have a higher catalyst reaction characteristic. Researches are being actively conducted for development of sensing materials having high sensitivity and selectivity by coupling various nanoparticle catalysts to the semiconductor metal oxide based sensing materials. These methods using the nanoparticle catalysts may include a chemical sensitization method and an electronic sensitization method on the basis of a principle. The chemical sensitization method may increase a concentration of gases used in the surface reaction by using a metal catalyst such as platinum (Pt) or gold (Au), thereby improving characteristics of the gas sensor. The electronic sensitization method may improve sensitivity of the gas sensor by using a variation of an oxidation number occurring by forming a metal (e.g., palladium (Pd), nickel (Ni), cobalt (Co), or silver (Ag)) into a metal oxide (e.g., PdO, NiO, Co2O3, or Ag2O).
Even though researches are conducted for the development of the various nanostructure and the sensing materials to which various nanoparticle catalysts are coupled, a semiconductor metal oxide based sensing material having very high sensitivity rapidly and accurately detecting a very small amount of a gas is not commercialized. Thus, a sensing material capable of selectively sensing a very small amount of a gas should be developed to realize a healthcare exhalation sensor.
A chemical deposition method, a physical deposition method, and a chemical growth method have been developed as conventional methods of synthesizing a nanostructure. However, these methods may include complex processes, and thus process costs and process times of these methods may be increased. In other words, these methods may be difficult to be applied to mass production.
In addition, nanoparticle catalysts should be uniformly distributed on an entire region of a sensing material to effectively increase sensitivity and selectivity of a sensor. A nanoparticle catalyst may be synthesized by a polyol synthesis method corresponding to a representative method of synthesizing a nanoparticle catalyst. In this case, aggregation between nanoparticle catalysts may be caused when the catalysts are coupled to a metal oxide material. Thus, it is difficult to uniformly distribute the catalysts on and in the sensing material.
To overcome the disadvantage described above, it is necessary to manufacture a nanoparticle catalyst having a size of several nm and to synthesize a nanostructure on which nanoparticle catalysts are uniformly distributed. In addition, it is necessary to develop a sensing material having a wide surface area reacting with gases by using a simple and effective manufacturing method. As a result, it is necessary to develop a material-synthesizing technique and a sensor-manufacturing technique which are capable of selectively sensing very small amounts of biomarker gases included in exhaled breath of a human body by satisfying the necessities described above.